This invention relates to sensor systems, and, more particularly, to a sensor system wherein an angularly dependent introduced error is corrected by a rigid body error-correcting element.
Optical sensors are used in aircraft and missile applications to receive radiated optical energy from a scene and convert the energy to an electrical signal. The electrical signal is provided to a display or further processed for pattern recognition or the like. The optical sensor and its related optical train, termed a sensor system, are usually packaged in a housing. The housing may be pivotably mounted within the airframe to allow the optical sensor to be pointed toward subjects of interest.
The sensor system is rather fragile and is easily damaged by dirt, erosion, chemicals, or high air velocity. The sensor system is therefore placed behind a window through which the sensor views the scene and which protects the sensor system from such external agents. The window must be transparent to the radiation of the operating wavelength of the sensor, resist attack from the external forces, and introduce minimal or correctable wavefront errors into the image received by the sensor.
For many applications such as low-speed aircraft and helicopters, the window may be spherical, with the sensor focal point placed at the center of the sphere to minimize gimbal angle-dependent bore sight and wavefront errors. However, in higher speed aircraft and missiles the spherical window is unsatisfactory, as it induces a great deal of aerodynamic drag that reduces the maximum speed and range of the vehicle. An elongated, relatively narrow window, termed a conformal window, is therefore preferred for use in high-speed applications to reduce the aerodynamic drag.
The nonspherical conformal window, while reducing aerodynamic drag, introduces wavefront errors into the optical beam which are dependent upon the angle of the line of sight of the sensor. These wavefront errors may lead to angularly dependent errors in identification and location of features in the field of regard of the sensor. Techniques exist for mathematically processing the sensor signal to reduce the impact of such introduced wavefront errors, but these techniques utilize large amounts of computer processing capability.
There is a need for an improved approach to sensor systems used with conformal windows and other wavefront error-introducing elements. Such an improved approach would reduce and, ideally, eliminate such wavefront errors of the optical beam reaching the sensor. The present invention fulfills this need, and further provides related advantages.
The present invention provides a sensor system that corrects known wavefront errors introduced by elements in the optical train such as conformal windows. The error correction is achieved in a passive manner, without the addition of any motor drives and without the use of computer processing of the sensor signal. The correction is tailored to the particular error-introducing element, and does not require any change in the basic optical system or motion control system. The present approach allows the use of further processing of the sensor signal to achieve further correction.
In accordance with the invention, a sensor system comprises a sensor, and an optical train adjustable to provide an optical beam to the sensor from a selected line of sight selected from any of a plurality of lines of sight. The optical train includes a wavefront error-introducing element in the optical train, such as a conformal window. The nature of the introduced error is a function of the selected line of sight. The optical train also includes a rigid-body wavefront error-correcting element in the optical train, which may be a refractive element or a reflective element. The rigid-body wavefront error-correcting element has a spatially dependent correction structure, with the nature of the correction being a function of the selected line of sight. The adjustment of the optical train to the selected line of sight moves the optical beam to the appropriate location of the rigid-body wavefront error-correcting element to correct for the corresponding introduced wavefront error of the wavefront error-introducing element at that selected line of sight.
In another aspect, a sensor system comprises a sensor, and an optical train that directs an optical beam from an external location to the sensor. The optical train comprises a wavefront error-introducing element having a plurality of wavefront error-introducing locations, with a known wavefront error associated with each selected wavefront error-introducing location. A rigid-body wavefront error-correcting element has a plurality of wavefront-correcting locations, with a known wavefront correction associated with each selected wavefront-correction location. A known wavefront correction location on the wavefront error-correction element corresponds to each known wavefront error at each selected wavefront error-introducing location. An optical beam-director element controllably directs the optical beam from the external location, through the known wavefront error-introducing location, through the known wavefront-correction location, and thence to the sensor. The passage of the optical beam through the wavefront correction location partially or completely corrects the error introduced by the passage through the error-introducing location.
The rigid-body wavefront error-correcting element may be fixed with respect to the wavefront error-introducing element. In another form of the invention, the wavefront error-correcting element may be affixed to the optical beam-director element or other movable element so as to achieve a coordinated movement of the wavefront error-correcting element with the changing of the line of sight of the optical train. In either case, the corrections required to correct the errors introduced by the error-introducing element as a function of the line of sight angular position are utilized in constructing the error-correcting element. The structure of the error-correcting element varies according to location, to correspond to the corrections required for each associated line of sight of the optical train.
Ideally, the wavefront error-correcting element completely corrects and negates the errors introduced by the wavefront error-introducing element. A complete correction may not be possible in all cases. Remaining errors may be corrected by mathematical processing of the sensor image signal or other techniques, as appropriate.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.